The fire is believed to have started when a caretaker switched on the lights, triggering a gas explosion. The caretaker was blown backwards by the blast but escaped unhurt. No one else was injured.
A US Air Force memorial library was destroyed but the main concern for conservationists is the collection of irreplaceable books, records and manuscripts housed in the county record office in the basement. The collection contains more than 2 million documents, including cathedral records dating back to 1090.
'It's a major disaster - the collection was of national significance and we don't know how much has been saved,' said Pearl Valentine, assistant director of Norfolk County Council's library and information service.
The county record office was not damaged by fire but water leaked into the basement, damaging the collection. Last night a fleet of removal vans ferried the documents to cold storage.
Tony Parker, a senior conservation officer at the British Library, said the first priority would be reducing the temperature and humidity of papers to prevent further damage.
The conservationists will only have between 24 and 48 hours before mould begins to grow. Water damage will already have begun distorting the paper and bindings and the inks will have run.
Mr Parker said: 'If it's not saturated then you can consider air drying or you can do it with blotting paper. Or you can freeze the lot and that buys you a lot of time because you can sit down and see the exact problem.'
Once frozen, the records can be stored until the best option for each item can be worked out. They can be freeze-dried - using a similar process to producing instant coffee, or they can be thawed and dried in a stream of air.
Once dried, the damaged leaves of paper may be stored between sheets of inert plastic.
'I would encapsulate it until somebody comes up with a good idea of how to deal with it. The approach is minimal intervention because anything you do changes the chemical composition of the paper,' said Mr Parker.
For exceptionally valuable items, leaf casting can be used. This replaces damaged areas of paper - tiny tears and abrasions - with new fibres which mesh with the old and strengthen the paper.
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